I would say au contrare! That is actually a very cool part of not believing in the silly part. I can have any themed party I want without worrying about pissing off this invisible friend or that invisible friend. I can also ignore them all - it's great to be free. Why would I "hate" someone that is just a fictional character?
Christmas really isn't a Christian holiday, originally, like so many other of their holidays. But regardless of which religion is the originator of the 25th of December celebrations - Christmas isn't about religion, anymore. For most people, whether religious or rational, Christmas is about being together with the family and having a ball. Xmas is also, usually, associated with peace and harmony.
I'm not a fan of celebrating things as trivial as date-related happenings. I don't care it's the 25th of December, my birthday is just as much of a day of life as the day afterwards is, and all other days in a year are. Even the seasons aren't what they used to be, so celebrating the first snowfall has become a thing of the past.
I prefer celebrating actual achievements, which did cost some effort. And, of course, it's good to remember significant achievements of the past, whether that is on the actual day of the year it once happened, or on any other day.
:Since 45 BCE, when the 25th of December was established in the Julian calendar as the winter solstice of Europe, (Latin: Bruma), the difference between the calendar year (365.2500 days) and the tropical year (365.2422 days) moved the day associated with the actual astronomical solstice forward approximately three days every four centuries until 1582 when Pope Gregory XIII changed the calendar bringing the northern winter solstice to around December 21. Yearly, in the Gregorian calendar, the solstice still fluctuates slightly but, in the long term, only about one day every 3000 years.:
The ancient Romans had a tree and gave presents too.
No one owns any day. I can celebrate any day any way I want.